Nitric oxide-dependent and -independent mechanisms in the relaxation elicited by acetylcholine in fetal rat aorta

Abstract

The aim of the present study was to analyze the mechanisms involved in the relaxation induced by 1 μM acetylcholine (ACh) in aortic segments from fetal rats at term precontracted with 3 μM prostaglandin F 2α (PGF 2α) and incubated with 1 μM indomethacin. The endothelium-dependent relaxation caused by ACh was reduced by the nitric oxide (NO) synthase inhibitor N G-monomethyl-L-arginine (L-NMMA, 0.1 mM), such an effect was reversed by 0.1 mM L-arginine (L-Arg). After precontraction of segments with 50 mM KCl the relaxant response to ACh was smaller than that after precontraction with PGF2 2α; this reduction was increased by L-NMMA, whereas L-NMMA plus L-Arg potentiated the relaxation. Thiopentone sodium (0.1 mM), ouabain (10 μM), tetraethylammonium (TEA, 0.5 mM) and apamin (1 μM), inhibitors of cytochrome P450 monooxygenases, Na + pump, Ca 2+-activated (K Ca) and small-conductance (SK Ca) K + channels, respectively, reduced the relaxation to ACh, which was unaffected by charybdotoxin (0.1 μM) and glibenclamide (1 μM), inhibitors of large-conductance BK Ca and ATP-sensitive K + channels. The L-NMMA/indomethacin-resistant relaxation to ACh was markedly reduced by thiopentone sodium, and similarly decreased by either ouabain or TEA. The endothelium-independent relaxation induced by exogenous NO (10 μM) in segments precontracted with PGF 2α was unaltered by ouabain, glibenclamide, TEA and after precontraction with 50 mM KCl, and potentiated by L-NMMA. The potentiation of NO responses by L-NMMA was also observed in segments precontracted with KCl. These results suggest that ACh relaxes the fetal rat aorta by endothelial release of both NO and endothelium-derived hyperpolarizing factor (EDHF), a metabolite derived from cytochrome P450 monooxygenases, that hyperpolarizes smooth muscle cells by activation of K Ca, essentially SK Ca channels, and Na + pump. It seems that when the effect of EDHF is abolished, the formation of NO could be increased.